Knee problems commonly occur in young people and adults. This
booklet contains general information about several knee problems. It includes
descriptions and a diagram of the different parts of the knee. Individual sections
of the booklet describe the symptoms, diagnosis, and treatment of specific
types of knee injuries and conditions. Information on how to prevent these
problems is also provided.

What Do the Knees Do? How Do They Work?

What Causes Knee Problems?

Joint Basics

What Are the Parts of the Knee?

How Are Knee Problems Diagnosed?

Knee Injuries and Problems

Arthritis

Cartilage Injuries and
Disorders

Injuries to the Meniscus

Ligament Injuries

Tendon Injuries and Disorders

Other Knee Injuries

What Kinds of Doctors Treat Knee Problems?

How Can People Prevent Knee Problems?

What Types of Exercise Are Most Suitable for Someone
With Knee Problems?

What Do the Knees Do? How Do They Work?

The knees provide stable support for the body and allow the legs to
bend and straighten. Both flexibility and stability are needed for standing and
for motions like walking, running, crouching, jumping, and turning.

Several kinds of supporting and moving parts, including bones,
cartilage, muscles, ligaments, and tendons, help the knees do their job. Any of
these parts can be involved in pain or dysfunction.

What Causes Knee Problems?

There are two general kinds of knee problems: mechanical and
inflammatory.

Mechanical Knee Problems

Some knee problems result from injury, such as a direct blow or
sudden movements that strain the knee beyond its normal range of movement.
Other problems, such as osteoarthritis in the knee, result from wear and tear
on its parts.

Inflammatory Knee Problems

Inflammation that occurs in certain rheumatic diseases, such as
rheumatoid arthritis and systemic lupus erythematosus,
can damage the knee.

Joint Basics

The point at which two or more bones are connected is called a
joint. In all joints, the bones are kept from grinding against each other by
padding called cartilage. Bones are joined to bones by strong, elastic bands of
tissue called ligaments. Tendons are tough cords of tissue that connect muscle
to bone. Muscles work in opposing pairs to bend and straighten joints. While
muscles are not technically part of a joint, they're important because strong
muscles help support and protect joints.

What Are the Parts of the Knee?

Like any joint, the knee is composed of bones and cartilage,
ligaments, tendons, and muscles (see the diagram).

Bones and Cartilage

The knee joint is the junction of three bones: the femur (thigh
bone or upper leg bone), the tibia (shin bone or larger bone of the lower leg),
and the patella (knee cap). The patella is 2 to 3 inches wide and 3 to 4 inches
long. It sits over the other bones at the front of the knee joint and slides
when the leg moves. It protects the knee and gives leverage to muscles.

The ends of the three bones in the knee joint are covered with articular cartilage, a tough, elastic material that helps
absorb shock and allows the knee joint to move smoothly. Separating the bones
of the knee are pads of connective tissue. One pad is called a meniscus (muh-NISS-kus). The plural is menisci (muh-NISS-sky).
The menisci are divided into two crescent-shaped discs positioned between the
tibia and femur on the outer and inner sides of each knee. The two menisci in
each knee act as shock absorbers, cushioning the lower part of the leg from the
weight of the rest of the body as well as enhancing stability.

Muscles

There are two groups of muscles at the knee. The quadriceps
muscle comprises four muscles on the front of the thigh that work to straighten
the leg from a bent position. The hamstring muscles, which bend the leg at the
knee, run along the back of the thigh from the hip to just below the knee.
Keeping these muscles strong with exercises such as walking up stairs or riding
a stationary bicycle helps support and protect the
knee.

Tendons and Ligaments

The quadriceps tendon connects the quadriceps muscle to the
patella and provides the power to extend the leg. Four ligaments connect the
femur and tibia and give the joint strength and stability:

The medial collateral ligament (MCL) provides stability
to the inner (medial) part of the knee.

The lateral collateral ligament (LCL) provides
stability to the outer (lateral) part of the knee.

The anterior cruciate
ligament (ACL), in the center of the knee, limits rotation and the forward
movement of the tibia.

The posterior cruciate
ligament (PCL), also in the center of the knee, limits backward movement
of the tibia.

Other ligaments are part of the knee capsule, which is a
protective, fiber-like structure that wraps around the knee joint. Inside the
capsule, the joint is lined with a thin, soft tissue called synovium.

How Are Knee Problems Diagnosed?

Doctors use several methods to diagnose knee problems.

Medical history--The patient tells the doctor details
about symptoms and about any injury, condition, or general health problem that
might be causing the pain.

Physical examination--The doctor bends, straightens,
rotates (turns), or presses on the knee to feel for injury and discover the
limits of movement and the location of pain. The patient may be asked to stand,
walk, or squat to help the doctor assess the knee's function.

Diagnostic tests--The doctor uses one or more tests to
determine the nature of a knee problem.

X ray (radiography)--An x-ray beam is passed
through the knee to produce a two-dimensional picture of the bones.

Computerized axial tomography (CAT) scan--X
rays lasting a fraction of a second are passed through the knee at
different angles, detected by a scanner, and analyzed by a computer. This
produces a series of clear cross-sectional images ("slices") of
the knee tissues on a computer screen. CAT scan images show soft tissues
such as ligaments or muscles more clearly than conventional x rays. The
computer can combine individual images to give a three-dimensional view of
the knee.

Bone scan (radionuclide scanning)--A very small amount of
radioactive material is injected into the patient's bloodstream and
detected by a scanner. This test detects blood flow to the bone and cell
activity within the bone and can show abnormalities in these processes
that may aid diagnosis.

Magnetic resonance imaging (MRI)--Energy from a powerful magnet
(rather than x rays) stimulates knee tissue to
produce signals that are detected by a scanner and analyzed by a computer.
This creates a series of cross-sectional images of a specific part of the
knee. An MRI is particularly useful for detecting soft tissue damage or
disease. Like a CAT scan, a computer is used to produce three-dimensional
views of the knee during MRI.

Arthroscopy--The doctor manipulates a small, lighted optic
tube (arthroscope) that has been inserted into
the joint through a small incision in the knee. Images of the inside of
the knee joint are projected onto a television screen. While the arthroscope is inside the knee joint, removal of loose
pieces of bone or cartilage or the repair of torn ligaments and menisci is
also possible.

Biopsy--The doctor removes tissue to examine under a
microscope.

Knee Injuries and Problems

Arthritis

What Is Arthritis of the Knee?

Arthritis of the knee is most often osteoarthritis. In this
disease, the cartilage in the joint gradually wears away. In rheumatoid
arthritis, which can also affect the knees, the joint becomes inflamed and
cartilage may be destroyed.* Arthritis not only affects joints; it can also
affect supporting structures such as muscles, tendons, and ligaments.

Osteoarthritis may be caused by excess stress on the joint from
deformity, repeated injury, or excess weight. It most often affects middle-aged
and older people. A young person who develops osteoarthritis may have an
inherited form of the disease or may have experienced continuous irritation
from an unrepaired torn meniscus or other injury.
Rheumatoid arthritis often affects people at an earlier age than osteoarthritis.

* The National Institute of Arthritis and Musculoskeletal and
Skin Diseases Information Clearinghouse has separate publications on
osteoarthritis, rheumatoid arthritis, and knee replacement. See the end of this
booklet for contact information.

Signs and Diagnosis

Someone who has arthritis of the knee may experience pain,
swelling, and a decrease in knee motion. A common symptom is morning stiffness
that lessens as the person moves around. Sometimes the joint locks or clicks
when the knee is bent and straightened, but these signs may occur in other knee
disorders as well. The doctor may confirm the diagnosis by performing a
physical examination and examining x rays, which
typically show a loss of joint space. Blood tests may be helpful for diagnosing
rheumatoid arthritis, but other tests may be needed too. Analyzing fluid from
the knee joint may be helpful in diagnosing some kinds of arthritis. The doctor
may use arthroscopy to directly see damage to cartilage, tendons, and ligaments
and to confirm a diagnosis, but arthroscopy is usually done only if a repair
procedure is to be performed.

Treatment

Most often osteoarthritis of the knee is treated with
pain-reducing medicines, such as aspirin or acetaminophen (Tylenol*); nonsteroidal anti-inflammatory drugs (NSAIDs),
such as ibuprofen (Motrin, Nuprin, Advil); and
exercises to restore joint movement and strengthen the knee. Losing excess
weight can also help people with osteoarthritis.

Rheumatoid arthritis of the knee may require physical therapy and
more powerful medications. In people with arthritis of the knee, a seriously
damaged joint may need to be replaced with an artificial one. (A new procedure
designed to stimulate the growth of cartilage by using a patient's own
cartilage cells is being used experimentally to repair cartilage injuries at
the end of the femur at the knee. It is not, however, a treatment for
arthritis.)

* Brand names included in this booklet are provided as examples
only, and their inclusion does not mean that these products are endorsed by the
National Institutes of Health or any other Government agency. Also, if a
particular brand name is not mentioned, this does not mean or imply that the
product is unsatisfactory.

Cartilage Injuries and Disorders

What Is Chondromalacia?

Chondromalacia (KON-dro-mah-LAY-she-ah),
also called chondromalaciapatellae, refers to
softening of the articular cartilage of the knee cap.
This disorder occurs most often in young adults and can be caused by injury,
overuse, parts out of alignment, or muscle weakness. Instead of gliding
smoothly across the lower end of the thigh bone, the knee cap rubs against it,
thereby roughening the cartilage underneath the knee cap. The damage may range
from a slightly abnormal surface of the cartilage to a surface that has been
worn away to the bone. Chondromalacia related to
injury occurs when a blow to the knee cap tears off either a small piece of
cartilage or a large fragment containing a piece of bone (osteochondral
fracture).

Symptoms and Diagnosis

The most frequent symptom is a dull pain around or under the knee
cap that worsens when walking down stairs or hills. A person may also feel pain
when climbing stairs or when the knee bears weight as it straightens. The
disorder is common in runners and is also seen in skiers, cyclists, and soccer
players. A patient's description of symptoms and a followup
x ray usually help the doctor make a diagnosis. Although arthroscopy can
confirm the diagnosis, it's not performed unless the condition requires
extensive treatment.

Treatment

Many doctors recommend that patients with chondromalacia
perform low-impact exercises that strengthen muscles, particularly the inner
part of the quadriceps, without injuring joints. Swimming, riding a stationary
bicycle, and using a cross-country ski machine are acceptable as long as the
knee doesn't bend more than 90 degrees. Electrical stimulation may also be used
to strengthen the muscles. If these treatments don't improve the condition, the
doctor may perform arthroscopic surgery to smooth the surface of the cartilage
and "wash out" the cartilage fragments that cause the joint to catch
during bending and straightening. In more severe cases, surgery may be
necessary to correct the angle of the knee cap and relieve friction with the
cartilage or to reposition parts that are out of alignment.

Injuries to the Meniscus

What Causes Injuries to the Meniscus?

The meniscus is easily injured by the force of rotating the knee
while bearing weight. A partial or total tear may occur when a person quickly
twists or rotates the upper leg while the foot stays still (for example, when
dribbling a basketball around an opponent or turning to hit a tennis ball). If
the tear is tiny, the meniscus stays connected to the front and back of the
knee; if the tear is large, the meniscus may be left hanging by a thread of
cartilage. The seriousness of a tear depends on its location and extent.

Symptoms

Generally, when people injure a meniscus, they feel some pain,
particularly when the knee is straightened. If the pain is mild, the person may
continue moving. Severe pain may occur if a fragment of the meniscus catches
between the femur and the tibia. Swelling may occur soon after injury if blood
vessels are disrupted, or swelling may occur several hours later if the joint
fills with fluid produced by the joint lining (synovium)
as a result of inflammation. If the synovium is
injured, it may become inflamed and produce fluid to protect itself. This makes
the knee swell. Sometimes, an injury that occurred in the past but was not
treated becomes painful months or years later, particularly if the knee is
injured a second time. After any injury, the knee may click, lock, or feel
weak. Although symptoms of meniscal injury may
disappear on their own, they frequently persist or
return and require treatment.

Diagnosis

In addition to listening to the patient's description of the
onset of pain and swelling, the doctor may perform a physical examination and
take x rays of the knee. The examination may include a test in which the doctor
bends the leg, then rotates the leg outward and inward
while extending it. Pain or an audible click suggests a meniscal
tear. An MRI may be recommended to confirm the diagnosis. Occasionally, the
doctor may use arthroscopy to help diagnose and treat a meniscal
tear.

Treatment

If the tear is minor and the pain and other symptoms go away, the
doctor may recommend a muscle-strengthening program. Exercises for meniscal problems are best started with guidance from a
doctor and physical therapist or exercise therapist. The therapist will make
sure that the patient does the exercises properly and without risking new or
repeat injury. The following exercises after injury to the meniscus are
designed to build up the quadriceps and hamstring muscles and increase
flexibility and strength.

Warming up the joint by riding a stationary bicycle,
then straightening and raising the leg (but not straightening it too
much).

Extending the leg while sitting (a weight may be worn
on the ankle for this exercise).

Raising the leg while lying on the stomach.

Exercising in a pool (walking as fast as possible in
chest-deep water, performing small flutter kicks while holding onto the
side of the pool, and raising each leg to 90 degrees in chest-deep water
while pressing the back against the side of the pool).

If the tear is more extensive, the doctor may perform
arthroscopic or open surgery to see the extent of injury and to repair the
tear. The doctor can sew the meniscus back in place if the patient is
relatively young, if the injury is in an area with a good blood supply, and if
the ligaments are intact. Most young athletes are able to return to active
sports after meniscus repair.

If the patient is elderly or the tear is in an area with a poor
blood supply, the doctor may cut off a small portion of the meniscus to even
the surface. In some cases, the doctor removes the entire meniscus. However,
osteoarthritis is more likely to develop in the knee if the meniscus is
removed. Medical researchers are investigating a procedure called an allograft,
in which the surgeon replaces the meniscus with one from a cadaver. A grafted
meniscus is fragile and will shrink and tear easily. Researchers have also
attempted to replace a meniscus with an artificial one, but this procedure is
even less successful than an allograft.

Recovery after surgical repair takes several weeks, and
postoperative activity is slightly more restricted than when the meniscus is
removed. Nevertheless, putting weight on the joint actually fosters recovery.
Regardless of the form of surgery, rehabilitation usually includes walking,
bending the legs, and doing exercises that stretch and build up leg muscles.
The best results of treatment for meniscal injury are
obtained in people who do not show articular
cartilage changes and who have an intact ACL.

Ligament Injuries

What Are the Causes of Anterior and Posterior Cruciate Ligament Injuries?

Injury to the cruciate ligaments is
sometimes referred to as a "sprain."* The ACL is most often stretched
or torn (or both) by a sudden twisting motion (for example, when the feet are
planted one way and the knees are turned another).

The PCL is most often injured by a direct impact, such as in an
automobile accident or football tackle.

* The National Institute of Arthritis and Musculoskeletal and
Skin Diseases Information Clearinghouse has a separate publication on sprains
and strains. See the end of this booklet for contact information.

Symptoms and Diagnosis

Injury to a cruciate ligament may not
cause pain. Rather, the person may hear a popping sound, and the leg may buckle
when he or she tries to stand on it. The doctor may perform several tests to
see whether the parts of the knee stay in proper position when pressure is
applied in different directions. A thorough examination is essential. An MRI is
very accurate in detecting a complete tear, but arthroscopy may be the only
reliable means of detecting a partial one.

Treatment

For an incomplete tear, the doctor may recommend that the patient
begin an exercise program to strengthen surrounding muscles. The doctor may
also prescribe a brace to protect the knee during activity. For a completely
torn ACL in an active athlete and motivated person, the doctor is likely to
recommend surgery. The surgeon may reattach the torn ends of the ligament or
reconstruct the torn ligament by using a piece (graft) of healthy ligament from
the patient (autograft) or from a cadaver
(allograft). Although synthetic ligaments have been tried in experiments, the
results have not been as good as with human tissue. One of the most important
elements in a patient's successful recovery after cruciate
ligament surgery is a 4- to 6-month exercise and rehabilitation program that
may involve using special exercise equipment at a rehabilitation or sports
center. Successful surgery and rehabilitation will allow the patient to return
to a normal lifestyle.

What Is the Most Common Cause of Medial and Lateral Collateral
Ligament Injuries?

The MCL is more easily injured than the LCL. The cause is most
often a blow to the outer side of the knee that stretches and tears the
ligament on the inner side of the knee. Such blows frequently occur in contact
sports like football or hockey.

Symptoms and Diagnosis

When injury to the MCL occurs, a person may feel a pop and the
knee may buckle sideways. Pain and swelling are common. A thorough examination
is needed to determine the kind and extent of the injury. To diagnose a
collateral ligament injury, the doctor exerts pressure on the side of the knee
to determine the degree of pain and the looseness of the joint. An MRI is
helpful in diagnosing injuries to these ligaments.

Treatment

Most sprains of the collateral ligaments will heal if the patient
follows a prescribed exercise program. In addition to exercise, the doctor may
recommend ice packs to reduce pain and swelling and a small sleeve-type brace
to protect and stabilize the knee. A sprain may take 2 to 4 weeks to heal. A
severely sprained or torn collateral ligament may be accompanied by a torn ACL,
which usually requires surgical repair.

Tendon Injuries and Disorders

What Causes Tendinitis and Ruptured
Tendons?

Knee tendon injuries range from tendinitis (inflammation of a tendon) to a ruptured (torn)
tendon. If a person overuses a tendon during certain activities such as
dancing, cycling, or running, the tendon stretches like a worn-out rubber band
and becomes inflamed. Also, trying to break a fall may cause the quadriceps
muscles to contract and tear the quadriceps tendon above the patella or the
patellar tendon below the patella. This type of injury is most likely to happen
in older people whose tendons tend to be weaker. Tendinitis
of the patellar tendon is sometimes called jumper's knee because in sports that
require jumping, such as basketball, the muscle contraction and force of
hitting the ground after a jump strain the tendon. After repeated stress, the
tendon may become inflamed or tear.

Symptoms and Diagnosis

People with tendinitis often have
tenderness at the point where the patellar tendon meets the bone. In addition,
they may feel pain during running, hurried walking, or jumping. A complete
rupture of the quadriceps or patellar tendon is not only painful, but also
makes it difficult for a person to bend, extend, or lift the leg against
gravity. If there is not much swelling, the doctor will be able to feel a
defect in the tendon near the tear during a physical examination. An x ray will
show that the patella is lower than normal in a quadriceps tendon tear and
higher than normal in a patellar tendon tear. The doctor may use an MRI to
confirm a partial or total tear.

Treatment

Initially, the doctor may ask a patient with tendinitis
to rest, elevate, and apply ice to the knee and to take medicines such as
aspirin or ibuprofen to relieve pain and decrease inflammation and swelling. If
the quadriceps or patellar tendon is completely ruptured, a surgeon will
reattach the ends. After surgery, the patient will wear a cast for 3 to 6 weeks
and use crutches. For a partial tear, the doctor might apply a cast without
performing surgery.

Rehabilitating a partial or complete tear of a tendon requires an
exercise program that is similar to but less vigorous than that prescribed for
ligament injuries. The goals of exercise are to restore the ability to bend and
straighten the knee and to strengthen the leg to prevent repeat injury. A
rehabilitation program may last 6 months, although the patient can return to
many activities before then.

What Causes Osgood-Schlatter
Disease?

Osgood-Schlatter disease is caused by
repetitive stress or tension on part of the growth area of the upper tibia (the
apophysis). It is characterized by inflammation of
the patellar tendon and surrounding soft tissues at the point where the tendon
attaches to the tibia. The disease may also be associated with an injury in
which the tendon is stretched so much that it tears away from the tibia and
takes a fragment of bone with it. The disease most commonly affects active
young people, particularly boys between the ages of 10 and 15, who play games
or sports that include frequent running and jumping.

Symptoms and Diagnosis

People with this disease experience pain just below the knee
joint that usually worsens with activity and is relieved by rest. A bony bump
that is particularly painful when pressed may appear on the upper edge of the
tibia (below the knee cap). Usually, the motion of the knee is not affected.
Pain may last a few months and may recur until the child's growth is completed.

Osgood-Schlatter disease is most often
diagnosed by the symptoms. An x ray may be normal, or show an injury, or, more
typically, show that the growth area is in fragments.

Treatment

Usually, the disease resolves without treatment. Applying ice to
the knee when pain begins helps relieve inflammation and is sometimes used
along with stretching and strengthening exercises. The doctor may advise the
patient to limit participation in vigorous sports. Children who wish to
continue moderate or less stressful sports activities may need to wear knee
pads for protection and apply ice to the knee after activity. If there is a great
deal of pain, sports activities may be limited until discomfort becomes
tolerable.

What Causes Iliotibial Band
Syndrome?

This is an overuse condition in which inflammation results when a
band of a tendon rubs over the outer bone (lateral condyle)
of the knee. Although iliotibial band syndrome may be
caused by direct injury to the knee, it is most often caused by the stress of
long-term overuse, such as sometimes occurs in sports training.

Symptoms and Diagnosis

A person with this syndrome feels an ache or burning sensation at
the side of the knee during activity. Pain may be localized at the side of the
knee or radiate up the side of the thigh. A person may also feel a snap when
the knee is bent and then straightened. Swelling is usually absent and knee motion
is normal. The diagnosis of this disorder is typically based on the symptoms,
such as pain at the outer bone, and exclusion of other conditions with similar
symptoms.

Treatment

Usually, iliotibial band syndrome
disappears if the person reduces activity and performs stretching exercises
followed by muscle-strengthening exercises. In rare cases when the syndrome
doesn't disappear, surgery may be necessary to split the tendon so it isn't
stretched too tightly over the bone.

Other Knee Injuries

What Is OsteochondritisDissecans?

Osteochondritisdissecans
results from a loss of the blood supply to an area of bone underneath a joint
surface and usually involves the knee. The affected bone and its covering of
cartilage gradually loosen and cause pain. This problem usually arises
spontaneously in an active adolescent or young adult. It may be due to a slight
blockage of a small artery or to an unrecognized injury or tiny fracture that
damages the overlying cartilage. A person with this condition may eventually develop
osteoarthritis.

Lack of a blood supply can cause bone to break down (avascular necrosis).* The
involvement of several joints or the appearance of osteochondritisdissecans in several family members may indicate that
the disorder is inherited.

* The National Institute of Arthritis and Musculoskeletal and
Skin Diseases Information Clearinghouse has a separate publication on avascular necrosis. See the end of this booklet for contact
information.

Symptoms and Diagnosis

If normal healing doesn't occur, cartilage separates from the
diseased bone and a fragment breaks loose into the knee joint, causing
weakness, sharp pain, and locking of the joint. An x ray, MRI, or arthroscopy
can determine the condition of the cartilage and can be used to diagnose osteochondritisdissecans.

Treatment

If cartilage fragments have not broken loose, a surgeon may fix
them in place with pins or screws that are sunk into the cartilage to stimulate
a new blood supply.

If fragments are loose, the surgeon may scrape down the cavity to
reach fresh bone and add a bone graft and fix the fragments in position.
Fragments that cannot be mended are removed, and the cavity is drilled or
scraped to stimulate new cartilage growth. Research is being done to assess the
use of cartilage cell and other tissue transplants to treat this disorder.

What Is Plica Syndrome?

Plica (PLI-kah)
syndrome occurs when plicae (bands of synovial tissue) are irritated by overuse or injury. Synovialplicae are the remains
of tissue pouches found in the early stages of fetal development.

As the fetus develops, these pouches normally combine to form one
large synovial cavity. If this process is incomplete,
plicae remain as four folds or bands of synovial tissue within the knee. Injury, chronic overuse,
or inflammatory conditions are associated with this syndrome.

Symptoms and Diagnosis

People with this syndrome are likely to experience pain and
swelling, a clicking sensation, and locking and weakness of the knee. Because
the symptoms are similar to those of some other knee problems, plica syndrome is often misdiagnosed. Diagnosis usually
depends on excluding other conditions that cause similar symptoms.

Treatment

The goal of treatment is to reduce inflammation of the synovium and thickening of the plicae.
The doctor usually prescribes medicine such as ibuprofen to reduce
inflammation. The patient is also advised to reduce activity, apply ice and an
elastic bandage to the knee, and do strengthening exercises. A cortisone
injection into the plica folds helps about half of those
treated. If treatment fails to relieve symptoms within 3 months, the doctor may
recommend arthroscopic or open surgery to remove the plicae.

What Kinds of Doctors Treat Knee Problems?

Extensive injuries and diseases of the knees are usually treated
by an orthopaedic surgeon, a doctor who has been
trained in the nonsurgical and surgical treatment of
bones, joints, and soft tissues such as ligaments, tendons, and muscles.
Patients seeking nonsurgical treatment of arthritis
of the knee may also consult a rheumatologist (a doctor specializing in the
diagnosis and treatment of arthritis and related disorders).

How Can People Prevent Knee Problems?

Some knee problems, such as those resulting from an accident,
can't be foreseen or prevented. However, a person can prevent many knee
problems by following these suggestions:

Before exercising or participating in sports, warm up
by walking or riding a stationary bicycle, then
do stretches. Stretching the muscles in the front of the thigh
(quadriceps) and back of the thigh (hamstrings) reduces tension on the
tendons and relieves pressure on the knee during activity.

Strengthen the leg muscles by doing specific exercises
(for example, by walking up stairs or hills, or by riding a stationary
bicycle). A supervised workout with weights is another way to strengthen
the leg muscles that support the knee.

Avoid sudden changes in the intensity of exercise.
Increase the force or duration of activity gradually.

Wear shoes that both fit properly and are in good
condition to help maintain balance and leg alignment when walking or
running. Knee problems can be caused by flat feet or overpronated
feet (feet that roll inward). People can often reduce some of these
problems by wearing special shoe inserts (orthotics).
Maintain a healthy weight to reduce stress on the knee. Obesity increases
the risk of degenerative (wearing) conditions such as osteoarthritis of
the knee.

What Types of Exercise Are Most Suitable for
Someone With Knee Problems?

Strengthening exercises help keep or increase muscle
strength. Strong muscles help support and protect joints affected by
arthritis.

Aerobic or endurance exercises improve function of
the heart and circulation and help control weight. Weight control can be
important to people who have arthritis because extra weight puts pressure
on many joints. Some studies show that aerobic exercise can reduce
inflammation in some joints.